2012 Volume 80 Issue 2 Pages 141-150
The soil contamination countermeasures law, implemented from February 2003, was revised in April 2010 to include contaminants of natural origins, and the development of immobilization technology for arsenic in natural soils is an urgent necessity. The authors reported that waste gypsum powder is effective for the immobilization of this kind of arsenic and suggested the precipitation of calcium arsenate by the reaction between calcium ions from gypsum and arsenate ions from soil. This study aimed to investigate the effect of gypsum on immobilization for soil samples with different arsenic dissolution properties and to check numerically the possibility of the precipitation of calcium arsenate by the calculation of its solubility. The concentration of arsenic dissolved from soils decreased to below the value of 0.01mg dm-3 (the environmental quality standards. e.q.s.) by the addition of gypsum powder from 0.5 to 5 %. The solubility of calcium arsenate was calculated to be 0.2 mg dm-3 as the lowest value in the pH range examined, which is much larger than the e.q.s., and the precipitation of calcium arsenate cannot be the cause of the immobilization. It was shown that gypsum promotes the adsorption of arsenate ions on soil particles due to adsorbed calcium ions. The adsorbed calcium ions are likely to neutralize the negative charge on soil, to provide positively charged adsorption sites for arsenate, and to promote the adsorption and fixation of negatively charged arsenate ions on soil particles.